Protein with recombinase activity for site-specific DNA-recombination

ABSTRACT

The invention relates to the use of a protein with recombinase activity to catalyze a site-specific DNA recombination and a method for producing a site-specific DNA recombination. The invention is applicable alone or in combination with other recombinase systems for genetic manipulation, for example in medical research. The objective of the invention is solved by the use of a protein with recombinase activity to catalyze a site-specific DNA recombination at, preferably at two, recognition sites that are identical or reverse complementary to each other. The invention also includes a method for producing a site-specific DNA recombination comprising the steps of a) providing a cell comprising at least two recognition sites that are identical or reverse complementary to each other; and b) contacting a protein with recombinase activity with the recognition sites, thereby producing the site-specific DNA-recombination.

The invention relates to the use of a protein with recombinase activity to catalyze a site-specific DNA recombination and a method for producing a site-specific DNA recombination. The invention is applicable alone or in combination with other recombinase systems for genetic manipulation, for example in medical research.

The use of site-specific DNA recombinases allows genetic manipulations in both prokaryotic and eukaryotic organisms. For this purpose, various site-specific DNA recombinases isolated from different organisms are used. The DNA recombination mediated by the site-specific recombinase occurs by cleavage and rejoining of DNA at specific DNA sequences, the so-called recognition sites (nucleic acid sequences of 10 to 150 base pairs). If two recognition sites are oriented in the same direction in a DNA strand, a nucleic acid segment flanked by the recognition sites is cut out (excision). If two recognition sequences flanking a nucleic acid segment in a DNA strand are oriented in the opposite direction, the site-specific DNA recombinase catalyzes the inversion of the nucleic acid segment. If two recombination sites are located on two different molecules, then the site-specific DNA recombinase catalyzes merge of two molecules (integration).

Among sites-specific recombinases a particular class called tyrosine recombinases (SSRs), such as Cre and Flp, has become an outstanding genetic tool. Unlike most SSRs, they do not require additional host factors for efficient catalysis and recognize relatively short sequences. Because of the simplicity and efficiency, these recombinases now serve as “molecular scissors” for robust, non-disruptive and reproducible genomic modifications.

The so-called Cre/loxP system (EP 0 2200 009 B1) is widely used in the prior art. Cre (amino acid sequence according to SEQ ID No. 4) is a site-specific DNA recombinase isolated from bacteriophage P1. The recognition site of the Cre protein is a nucleotide sequence of 34 base pairs, the so-called loxP site (SEQ ID No. 5). Cre shows high recombinase activity both in bacterial and mammalian cells. It is known to modify the amino acid sequence of Cre in order to obtain novel site-specific recombinases (DE 102 07 313 A1).

Another site-specific DNA recombinase system is the so-called Flp/FRT system isolated from Saccharomyces cerevisiae. The Flp/FRT system includes the recombinase Flp (flippase) (amino acid sequence according to SEQ ID No. 6) that catalyzes DNA-recombination on its recognition sites, the so-called FRT sites (SEQ ID No. 7).

In addition to the Cre/loxP and the Flp/FRT system, that are both the most widely used site-specific recombinase systems of tyrosine class, other recombinase systems are known in the art. U.S. Pat. No. 7,422,889 and U.S. Pat. No. 7,915,037 B2 disclose the so-called Dre/rox system that comprises a Dre recombinase (amino acid sequence according to SEQ ID No. 8) isolated from Enterobacteria phage D6, the recognition site of which is called rox-site (SEQ ID No. 9). Further known recombinase systems are the VCre/VloxP system isolated from Vibrio plasmid p0908 (amino acid sequence according to SEQ ID No. 10 for the recombinase, and SEQ ID No. 11 for the VloxP site), and the sCre/SloxP system (WO 2010/143606 A1; Suzuki and Nakayama, 2011).

The known recombinase systems that are known in the art show different activities in cells of different origin. For many applications, such as the production of transgenic animals with conditional gene knockouts, two or more recombinase systems are used in combination with each other. However, emerging complex genetics studies and applications require simultaneous use of multiple recombinases. At the same time not all well-described site-specific recombinases are equally applicable in all model organisms due to, e.g. genome specificity (off-target activity on cryptic recognition target sites). For that reason it is important that an optimal recombinase can be chosen depending on the target organism or experimental setup. Therefore, there is a need for the provision of alternative recombinase systems that can be used to catalyze a site-specific DNA recombination on short targets in a variety of cell types and with high activity and with low toxicity.

It is the objective of the invention to provide novel recombinase systems for site-specific genetic recombination that can be used in a variety of cell types. Another object of the invention is to provide a novel, highly specific, recombinase system for site-specific genetic recombination with preferably low toxicity.

The objective is solved by the use of a protein with recombinase activity, wherein the protein comprises an amino acid sequence exhibiting at least 70%, preferably at least 80%, preferably at least 90%, particularly preferred at least 95%, even more preferred at least 99% amino acid sequence identity to SEQ ID No. 1 to catalyze a site-specific DNA recombination at, preferably at two, recognition sites that are identical or reverse complementary to each other, wherein at least one recognition site comprises a nucleic acid sequence according to or reverse complementary to SEQ ID No. 2; or a nucleic acid sequence that is a functional mutant thereof.

The protein with recombinase activity as defined above and used in the invention protein is referred to herein as “Vika”. The recognition site of the site-specific recombinase Vika is referred to herein as “vox-site” or simply “vox”. A vox-site is characterized by its nucleic acid sequence according to SEQ ID No. 2 or a nucleic acid sequence reverse complementary thereto. Recognition sites that exhibit a nucleic acid sequence identity to SEQ ID No. 2 of at least 70%, preferably at least 80%, preferably at least 90%, particularly preferred at least 95%, even more preferred at least 99%, or nucleic acid sequences reverse complementary thereto; and that are targets for the site specific recombination of Vika (herein referred to as “functional mutants” of vox sites) are also “vox-sites” within the sense of the invention. However, particularly preferred vox-sites exhibit a nucleic acid sequence according to SEQ ID No. 2 or a nucleic acid sequence reverse complementary thereto (herein also referred to as “wild type vox-site”).

By a functional mutant of the wild type vox-site is meant that one or more nucleic acids are added to, inserted, deleted or substituted from the nucleic acid sequence according to SEQ ID No. 2. At the same time, the functional mutant of the recognition site with a nucleic acid sequence according to SEQ ID No. 2 exhibits a nucleic acid sequence identity to SEQ ID No. 2 of at least 70%, preferably at least 80%, preferably at least 90%, particularly preferred at least 95%, even more preferred at least 99% and is a functional recognition site of Vika. Preferred mutations are point mutations or an exchange of the spacer region of the recognition site according to SEQ ID No. 2. It is known that the exchange of a spacer region of a recognition site does not influence its activity as a target site for the specific recombinase. Therefore, particularly preferred functional mutants of the wild type vox-site exhibit a nucleic acid sequence identity of at least 70%, preferably at least 75%, to the nucleic acid sequence according to SEQ ID No. 2 or its reverse complementary sequence; and exhibit mutations in the nucleic acids 14 to 21 (spacer region) of SEQ ID No. 2.

The invention is based on the finding of the inventors that the protein ZP_(—)05884863 (http://www.ncbi.nlm.nih.gov/protein/ZP_(—)05884863, SEQ ID No. 1), which is referred to herein as Vika (wild type), shows a Cre recombinase-like activity and recognizes recognition sites according to SEQ ID No. 2. Vika is derived from the gram-negative bacterium Vibrio coralliilyticus ATCC BAA 450. The amino acid sequences of Cre and Vika show a low identity of merely 26% (49% sequence similarity). The recognition sequences for Cre and Vika show a low identity of 35.6% (35.6% sequence similarity).

The inventors identified six putative recognition sites with a lox-like structure in Vibrio coralliilyticus. Upon extensive studies only one thereof, the wild type vox site, turned out to be the actual recognition site of Vika. The wild type vox-site is a 34 bp region of two inverted repeats that comprises about 50% sequence homology to the loxP site. By expression of the nucleic acid sequence encoding for Vika (SEQ ID No. 3) in E. coli, the Vika protein could be successfully obtained and its recombinase activity and specifity for the wild type vox site could be demonstrated. Vika was shown to belong to tyrosine class of SSRs and therefore, as further demonstrated in experiments, does not require expression of auxiliary factors for enzymatic activity in various cells types. Furthermore, it was shown that Vika does not cross react with other known recombinase systems and is superior in the activity compared to some of the known recombinase systems, at least in certain cell types.

The invention is based further on the finding that the protein WP_(—)008927019.1 (http://www.ncbi.nlm.nih.gov/protein/WP_(—)008927019.1, SEQ ID No. 35), which is referred to herein as Panto (wild type), shows a Cre recombinase-like activity and recognizes recognition sites according to SEQ ID No. 36. Panto is derived from the enterobacterium Panteo sp. aB. The amino acid sequences of Cre and Panto show a low identity of 41% (57% sequence similarity).

The invention is based further on the finding that the protein YP_(—)004250912. (http://www.ncbi.nlm.nih.gov/protein/YP_(—)004250912.1, SEQ ID No. 37), which is referred to herein as Nigri (wild type), which was already predicted to be a putatice Cre-like recombinase, recognizes recognition sites according to SEQ ID No. 38. Nigri is derived from the gram-negative bacterium Vibrio nigripulchritudo. The recognition sequences for Cre and Nigri show a low identity of 34.7% (34.7% sequence similarity).

The inventors identified recognition sites for both Panto and Nigri.

The objective is further solved by the use of a protein with recombinase activity, wherein the protein comprises an amino acid sequence exhibiting at least 70%, preferably at least 80%, preferably at least 90%, particularly preferred at least 95%, even more preferred at least 99% amino acid sequence identity to SEQ ID No. 35 to catalyze a site-specific DNA recombination at, preferably at two, recognition sites that are identical or reverse complementary to each other, wherein at least one recognition site comprises a nucleic acid sequence according to or reverse complementary to SEQ ID No. 35; or a nucleic acid sequence that is a functional mutant thereof.

The protein with recombinase activity as defined above and used in the invention protein is referred to herein as “Panto”. The recognition site of the site-specific recombinase Panto is referred to herein as “pox-site” or simply “pox”. A pox-site is characterized by its nucleic acid sequence according to SEQ ID No. 36 or a nucleic acid sequence reverse complementary thereto. Recognition sites that exhibit a nucleic acid sequence identity to SEQ ID No. 36 of at least 70%, preferably at least 80%, preferably at least 90%, particularly preferred at least 95%, even more preferred at least 99%, or nucleic acid sequences reverse complementary thereto; and that are targets for the site specific recombination of Panto (herein referred to as “functional mutants” of pox sites) are also “pox-sites” within the sense of the invention. However, particularly preferred pox-sites exhibit a nucleic acid sequence according to SEQ ID No. 36 or a nucleic acid sequence reverse complementary thereto (herein also referred to as “wild type pox-site”).

By a functional mutant of the wild type pox-site is meant that one or more nucleic acids are added to, inserted, deleted or substituted from the nucleic acid sequence according to SEQ ID No. 36. At the same time, the functional mutant of the recognition site with a nucleic acid sequence according to SEQ ID No. 36 exhibits a nucleic acid sequence identity to SEQ ID No. 36 of at least 70%, preferably at least 80%, preferably at least 90%, particularly preferred at least 95%, even more preferred at least 99% and is a functional recognition site of Panto. Preferred mutations are point mutations or an exchange of the spacer region of the recognition site according to SEQ ID No. 36. It is known that the exchange of a spacer region of a recognition site does not influence its activity as a target site for the specific recombinase. Therefore, particularly preferred functional mutants of the wild type pox-site exhibit a nucleic acid sequence identity of at least 70%, preferably at least 75%, to the nucleic acid sequence according to SEQ ID No. 36 or its reverse complementary sequence.

The objective is further solved by the use of a protein with recombinase activity, wherein the protein comprises an amino acid sequence exhibiting at least 70%, preferably at least 80%, preferably at least 90%, particularly preferred at least 95%, even more preferred at least 99% amino acid sequence identity to SEQ ID No. 37 to catalyze a site-specific DNA recombination at, preferably at two, recognition sites that are identical or reverse complementary to each other, wherein at least one recognition site comprises a nucleic acid sequence according to or reverse complementary to SEQ ID No. 37; or a nucleic acid sequence that is a functional mutant thereof.

The protein with recombinase activity as defined above and used in the invention protein is referred to herein as “Nigri”. The recognition site of the site-specific recombinase Nigri is referred to herein as “nox-site” or simply “nox”. A nox-site is characterized by its nucleic acid sequence according to SEQ ID No. 38 or a nucleic acid sequence reverse complementary thereto. Recognition sites that exhibit a nucleic acid sequence identity to SEQ ID No. 38 of at least 70%, preferably at least 80%, preferably at least 90%, particularly preferred at least 95%, even more preferred at least 99%, or nucleic acid sequences reverse complementary thereto; and that are targets for the site specific recombination of Nigri (herein referred to as “functional mutants” of nox sites) are also “nox-sites” within the sense of the invention. However, particularly preferred nox-sites exhibit a nucleic acid sequence according to SEQ ID No. 38 or a nucleic acid sequence reverse complementary thereto (herein also referred to as “wild type nox-site”).

By a functional mutant of the wild type nox-site is meant that one or more nucleic acids are added to, inserted, deleted or substituted from the nucleic acid sequence according to SEQ ID No. 38. At the same time, the functional mutant of the recognition site with a nucleic acid sequence according to SEQ ID No. 38 exhibits a nucleic acid sequence identity to SEQ ID No. 38 of at least 70%, preferably at least 80%, preferably at least 90%, particularly preferred at least 95%, even more preferred at least 99% and is a functional recognition site of Nigri. Preferred mutations are point mutations or an exchange of the spacer region of the recognition site according to SEQ ID No. 38. It is known that the exchange of a spacer region of a recognition site does not influence its activity as a target site for the specific recombinase. Therefore, particularly preferred functional mutants of the wild type nox-site exhibit a nucleic acid sequence identity of at least 70%, preferably at least 75%, to the nucleic acid sequence according to SEQ ID No. 38 or its reverse complementary sequence.

Within the sense of the invention the terms “site-specific DNA recombinase” and “recognition site” are used as defined above in the discussion of the prior art.

The invention also includes a method for producing a site-specific DNA recombination. The method according to the invention comprises the steps of

-   a) providing a cell comprising at least two recognition sites that     are identical or reverse complementary to each other, wherein at     least one recognition site comprises a nucleic acid sequence     according to or reverse complementary to SEQ ID No. 2; or a nucleic     acid sequence that is a functional mutant thereof; and -   b) contacting a protein with recombinase activity, wherein the     protein exhibits an amino acid sequence of at least 70%, preferably     at least 80%, preferably at least 90%, particularly preferred at     least 95%, even more preferred at least 99%, amino acid sequence     identity to SEQ ID No. 1 with the recognition sites, thereby     producing the site-specific DNA-recombination.

In this method according to the invention the Vika protein is contacted with at least two vox sites preferably inside a cell. Upon binding of the Vika protein to the vox sites, site-specific DNA-recombination occurs.

The invention further includes a method for producing a site-specific DNA recombination using Panto or Nigri. The method according to the invention comprises the steps of

-   a) providing a cell comprising at least two recognition sites that     are identical or reverse complementary to each other, wherein at     least one recognition site comprises a nucleic acid sequence     according to or reverse complementary to SEQ ID No. 36 or 38; or a     nucleic acid sequence that is a functional mutant thereof; and -   b) contacting a protein with recombinase activity, wherein the     protein exhibits an amino acid sequence of at least 70%, preferably     at least 80%, preferably at least 90%, particularly preferred at     least 95%, even more preferred at least 99%, amino acid sequence     identity to SEQ ID No. 35 or 37 with the recognition sites, thereby     producing the site-specific DNA-recombination.

In this method according to the invention the Panto protein is contacted with at least two pox sites inside a cell. Alternatively the Nigri protein is contacted with at least two nox sites inside a cell. Upon binding of the Panto or Nigri protein to the pox sites or respectively to the nox sites, site-specific DNA-recombination occurs.

The method according to the invention can be carried out in vitro or in vivo. In case the invention is carried out in an animal (including humans) it is preferably carried out for non-therapeutic use. The method is applicable in all areas where state of the art site specific recombinases are conventionally used (including inducible knock out or knock in mice and other transgenic animal models). In preferred methods according to the invention, the site-specific recombination results in integration, deletion, inversion, translocation or exchange of DNA. Preferably, the method according to the invention is not used for the therapeutic treatment of a human being or an animal. Preferably the method according to the invention is used to create animal models, which are useful for biomedical research, e.g. as models for human diseases.

In a method according to the invention, the nucleic acid sequence encoding for Vika (or Panto or Nigri respectively) is either already present in the cell or introduced into the cell, preferably by recombinant techniques. This preferred method according to the invention further includes the step of

-   c) introducing into the cell a nucleic acid encoding for the Vika     protein (or the Panto or Nigri protein respectively) with     recombinase activity, wherein said nucleic acid encoding for the     Vika protein preferably exhibits at least 70%, preferably at least     80%, preferably at least 90%, particularly preferred at least 95%,     even more preferred at least 99% nucleic acid sequence identity to     SEQ ID No. 3.

For activation of the expression of the nucleic acid encoding for Vika (or Panto or Nigri respectively), the nucleic acid encoding for Vika (or Panto or Nigri respectively) further comprises a regulatory nucleic acid sequence, preferably a promoter region. Hence, expression of the nucleic acid encoding for the protein with recombinase activity is produced by activating the regulatory nucleic acid sequence. Accordingly, to induce a DNA recombination, the regulatory nucleic acid sequence (preferably the promoter region) is activated to express the gene encoding for the Vika protein (or the Panto or Nigri protein respectively). Preferably, the regulatory nucleic acid sequence (preferably the promoter region) is either introduced into the cell in the method of the invention, preferably together with the sequence encoding for Vika (or Panto or Nigri respectively), or the regulatory nucleic acid sequence is already present in the cell in the beginning of the method according to the invention. In the second case, merely the nucleic acid encoding for the Vika (or Panto or Nigri respectively) protein is introduced into the cell (and placed under the control of the regulatory nucleic acid sequence).

By the term “regulatory nucleic acid sequences” within the sense of the invention gene regulatory regions of DNA are meant. In addition to promoter regions the term encompasses operator regions more distant from the gene as well as nucleic acid sequences that influence the expression of a gene, such as cis-elements, enhancers or silencers. The term “promoter region” within the sense of the invention refers to a nucleotide sequence on the DNA allowing a regulated expression of a gene. In this case the promoter region allows regulated expression of the nucleic acid encoding for Vika (or Panto or Nigri respectively).

The promoter region is located at the 5′-end of the gene and thus before the RNA coding region. Both, bacterial and eukaryotic promoters are applicable for the invention.

In a method according to the invention, the vox sites (or pox sites or nox sites respectively) are either included in the cell or introduced into the cell, preferably by recombinant techniques. This preferred method according to the invention further includes the steps of introducing into a cell the following nucleic acids:

-   a) a first nucleic acid (first recognition site, first vox site or     first pox site or first nox site respectively) comprising a nucleic     acid sequence according to or reverse complementary to SEQ ID No. 2     (or SEQ ID No. 36 or 38 respectively); or a nucleic acid sequence     that is a functional mutant thereof; -   b) a second nucleic acid (second recognition site, second vox site     or second pox site or second nox site respectively) comprising a     nucleic acid sequence identical or reverse complementary to the     nucleic acid sequence of the first nucleic acid (first recognition     site).

In a preferred method according to the invention, a nucleic acid encoding for the Vika (or Panto or Nigri respectively), protein and one or two, preferably two, vox sites (or pox or nox sites respectively), are introduced into the cell. This method includes the following steps:

-   -   introducing into a cell the following nucleic acids:         -   i) a nucleic acid encoding for Vika (or Panto or Nigri             respectively), wherein the nucleic acid is introduced into             the DNA such, that a regulatory nucleic acid sequence             (preferably a promoter region) controls the expression of             the nucleic acid encoding for Vika (or Panto or Nigri             respectively),         -   ii) a nucleic acid (first recognition site, first vox site             or first pox site or first nox site respectively) comprising             a nucleic acid sequence according to or reverse             complementary to SEQ ID No. 22 (or SEQ ID No. 36 or 38             respectively); or a nucleic acid sequence that is a             functional mutant thereof;         -   iii) a nucleic acid (second recognition site, second vox             site or second pox site or second nox site respectively)             comprising a nucleic acid sequence identical or reverse             complementary to the nucleic acid sequence defined in ii)             (nucleic acid sequence of the first recognition site), and     -   activating the regulatory nucleic acid sequence (preferably the         promoter region) to induce expression of the first nucleic acid         for the synthesis of the protein with recombinase activity.

By this preferred method according to the invention, the nucleic acid sequence encoding for Vika (or Panto or Nigri respectively), is introduced into a cell and at least two recognition sizes (vox sites or pox sites or nox sites respectively) are introduced into the genomic or episomal DNA of the cell. The steps i) to iii) can be performed in arbitrary order.

The introduction of the nucleic acids into the cells is performed using techniques of genetic manipulation known by a person skilled in the art. Among suitable methods are cell transformation bacterial cells and transfection or viral infection for mammalian cells, whereby a nucleic acid sequence encoding the protein is introduced into the cell as a component of a vector or part of virus-encoding DNA or RNA. The cell culturing is carried out by methods known to a person skilled in the art for the culture of the respective cells. Therefore, cells are preferably transferred into a conventional culture medium, and cultured at temperatures (preferably 35-38° C.) and in a gas atmosphere that is conducive to the survival of the cells.

The method according to the invention can be performed using eukaryotic and prokaryotic cells; preferred prokaryotic cells are bacterial cells. Preferred prokaryotic cells are cells of Escherichia coli. Preferred eukaryotic cells are yeast cells (preferably Saccharomyces cerevisiae), insect cells, non-insect invertebrate cells, amphibian cells, or mammalian cells (preferably somatic or pluripotent stem cells, including embryonic stem cells and other pluripotent stem cells, like induced pluripotent stem cells, and other native cells or established cell lines, including NIH3T3, CHO, HeLa, HEK293, hiPS). In case of human embryonic stem cells, cells are preferably obtained without destructing human embryos, e.g. by outgrowth of single blastomeres derived from blastocysts as described by (Chung 2008), by parthenogenesis, e.g. from a one-pronuclear oocyte as described by (Lin 2007) or by parthenogenetic activation of human oocytes as described by (Mai 2007). Also preferred are cells of a non-human host organism, preferably non-human germ cells, somatic or pluripotent stem cells, including embryonic stem cells, or blastocytes.

Further, the invention includes a nucleic acid comprising a nucleic acid sequence according to or reverse complementary to SEQ ID No. 2 (or SEQ ID No. 36 or SEQ ID No. 38), or a nucleic acid sequence that is a functional mutant thereof A nucleic acid according to the invention comprises a maximum of 40, preferably 34, base pairs. This nucleic acid according to the invention includes the recognition site vox of the Vika protein (or the pox site of Panto or the nox site of Nigri respectively) used according to the invention. Further the invention includes a vector (also referred to herein as “reporter vector”) comprising at least one nucleic acid comprising a nucleic acid sequence according to or reverse complementary to SEQ ID No. 2 (or SEQ ID No. 36 or SEQ ID No. 38), or a nucleic acid sequence that is a functional mutant thereof. In a preferred embodiment of the invention the vector comprises at least two vox-sites (or at least two pox sites or at least two nox sites respectively), i.e. at least two nucleic acids that independently of each other exhibit a nucleic acid sequence according to or reverse complementary to SEQ ID No. 2 (or SEQ ID No. 36 or SEQ ID No. 38), or a nucleic acid sequence that is a functional mutant thereof. Thereby the at least two vox-sites are preferably not located consecutively in the vector. Rather the at least two vox sites (or at least two pox sites or at least two nox sites respectively), are positioned such that they are flanking a DNA segment, that upon recognition of the vox sites by the Vika protein (or the pox sites by Panto or the nox sites by Nigri respectively), the DNA segment is either excised or inverted. Thereby the DNA segment can preferably contain a gene or a promoter region. As described above, the DNA segment is excised when it is flanked by two vox-sites of the same orientation (same nucleic acid sequence). An inversion of the DNA segment is catalyzed by the Vika protein (or Panto or Nigri respectively), when the DNA segment is flanked by two vox-sites (or pox sites or nox sites respectively), arranged in opposite orientations (i.e. comprise a nucleic acid sequence reverse complementary to one another).

The term “nucleic acids” as used herein includes not only deoxyribonucleic acids (DNA) and ribonucleic acids (RNA), but also all other linear polymers in which the bases adenine (A), cytosine (C), guanine (G) and thymine (T) or uracil (U) are arranged in a corresponding sequence (nucleic acid sequence). The invention also comprises the corresponding RNA sequences (in which thymine is replaced by uracil), complementary sequences and sequences with modified nucleic acid backbone or 3′ or 5′-terminus. Nucleic acids in the form of DNA are however preferred.

The term reporter vector as used herein includes a plasmid, virus or other nucleic acid carriers, that comprise a nucleic acid sequence according to the invention by genetic recombination (recombinantly), e.g. by insertion or incorporation of said nucleic acid sequence. Prokaryotic vectors as well as eukaryotic vectors, for example artificial chromosomes, such as YAC (yeast artificial chromosomes), are applicable for the invention. Typically, the expression vector comprises an origin of replication, a promoter, as well as specific gene sequences that allow phenotypic selection of host cells comprising the reporter vector.

The invention also includes a nucleic acid that encodes for a protein with recombinase activity, preferably Vika (or Panto or Nigri respectively), wherein the protein preferably comprises an amino acid sequence exhibiting at least 70%, preferably at least 80%, preferably at least 90%, particularly preferred at least 95%, even more preferred at least 99% amino acid sequence identity to SEQ ID No. 1 (or SEQ ID No. 35 or 37 respectively). Particularly preferred is a nucleic acid encoding for the Vika protein (or Panto or Nigri respectively), used according to the invention with a nucleic acid sequence according to SEQ ID No. 1 (or SEQ ID No. 35 or 37 respectively). Preferably, the nucleic acid comprises a nucleic acid sequence exhibiting at least 70%, preferably at least 80%, preferably at least 90%, particularly preferred at least 95%, even more preferred at least 99% nucleic acid sequence identity to SEQ ID No. 3. Furthermore, the invention includes a vector comprising said nucleic acid according to the invention (encoding for the protein with recombinase activity).

The invention also includes the use of the nucleic acids or vectors according to the invention in a method according to the invention for producing a site-specific DNA recombination.

When a nucleic acid encoding for a Vika protein, in particular in the form of a vector according to the invention, and at least two vox-sites as recognition sites are introduced into a host cell, a site-specific recombination is catalyzed upon expression of the protein Vika by its recognition of the vox sites.

Similar when a nucleic acid encoding for a Panto or Nigri protein, in particular in the form of a vector according to the invention, and at least two pox-sites or nox-sites as recognition sites are introduced into a host cell, a site-specific recombination is catalyzed upon expression of the protein Panto or Nigri by its recognition of the pox sites or nox sites respectively.

Accordingly, the invention also includes an isolated host cell comprising the following recombinant DNA fragments:

-   -   at least one, preferably at least two, nucleic acids according         to the invention comprising a vox-site (preferably two nucleic         acids according to the invention that include a vox-site,         respectively, flank a further DNA segment) and/or a nucleic acid         according to the invention encoding for a Vika protein or     -   a vector according to the invention comprising at least two         nucleic acids comprising a vox-site (preferably two nucleic         acids according to the invention that include a vox-site,         respectively, flank a further DNA segment) and/or a vector         according to the invention comprising a nucleic acid encoding         for a Vika-protein.

Alternatively, the invention includes an isolated host cell comprising the following recombinant DNA fragments:

-   -   at least one, preferably at least two, nucleic acids according         to the invention comprising a pox-site (preferably two nucleic         acids according to the invention that include a pox-site,         respectively, flank a further DNA segment) and/or a nucleic acid         according to the invention encoding for a Panto protein or     -   a vector according to the invention comprising at least two         nucleic acids comprising a pox-site (preferably two nucleic         acids according to the invention that include a pox-site,         respectively, flank a further DNA segment) and/or a vector         according to the invention comprising a nucleic acid encoding         for a Panto-protein,     -   or     -   at least one, preferably at least two, nucleic acids according         to the invention comprising a nox-site (preferably two nucleic         acids according to the invention that include a nox-site,         respectively, flank a further DNA segment) and/or a nucleic acid         according to the invention encoding for a Nigri protein or     -   a vector according to the invention comprising at least two         nucleic acids comprising a nox-site (preferably two nucleic         acids according to the invention that include a nox-site,         respectively, flank a further DNA segment) and/or a vector         according to the invention comprising a nucleic acid encoding         for a Nigri-protein.

The invention concerns only those isolated host cells that comprise the above mentioned nucleic acids or vectors recombinantly and not naturally, i.e. by genetic modification of the host cell. In particular the invention does not include cells of the organism Vibrio coralliilyticus ATCC BAA-450 that naturally contain a nucleic acid sequence encoding for Vika and comprising the recognition site vox. Further, the invention does preferably not include cells of the organism Pantoea sp. aB or Vibrio nigripulchritudo that contain a nucleic acid sequence encoding for Panto or Nigri and comprising the recognition site pox or nox.

Particularly preferred are isolated host cells that contain both, a nucleic acid encoding for Vika and at least two vox-sites (which are either oriented in the same or in opposite direction). Further preferred are isolated host cells that contain both, a nucleic acid encoding for Panto and at least two pox-sites (which are either oriented in the same or in opposite direction) or encoding for Nigri and at least two nox-sites (which are either oriented in the same or in opposite direction).

A host cell within the sense of the invention is a naturally occurring cell or a (optionally transformed or genetically modified) cell line that comprises at least one vector according to the invention or a nucleic acid according to the invention recombinantly, as described above. Thereby, the invention includes transient transfectants (e.g. by mRNA injection) or host cells that include at least one expression vector according to the invention as a plasmid or artificial chromosome, as well as host cells in which an expression vector according to the invention is stably integrated into the genome of said host cell. The host cell is preferably selected from cells of prokaryotes and eukaryotes. Preferred prokaryotic cells are cells of Escherichia coli. Preferred eukaryotic cells are selected from yeast cells (preferably Saccharomyces cerevisiae), insect cells, non-insect invertebrate cells, amphibian cells, and mammalian cells (preferably embryonal stem cells, NIH3T3, CHO, HeLa, HEK293, hiPS). Embryonal stem cells derived by killing of human embryos are specifically excluded from the term “host cells” within the sense of the invention.

With the invention it is also possible to induce tissue-specific site-specific recombination in non-human host organisms, such as mammals Therefore the invention also includes a non-human host organism comprising the following recombinant DNA fragments:

-   -   at least one, preferably at least two, nucleic acids according         to the invention comprising a vox-site (preferably two nucleic         acids according to the invention that include a vox-site,         respectively, flank a further DNA segment) and/or a nucleic acid         according to the invention encoding for a Vika protein or     -   a vector according to the invention comprising at least two         nucleic acids comprising a vox-site (preferably two nucleic         acids according to the invention that include a vox-site,         respectively, flank a further DNA segment) and/or a vector         according to the invention comprising a nucleic acid encoding         for a Vika-protein;     -   or     -   at least one, preferably at least two, nucleic acids according         to the invention comprising a pox or nox-site (preferably two         nucleic acids according to the invention that include a pox-site         or nox-site, respectively, flank a further DNA segment) and/or a         nucleic acid according to the invention encoding for a Panto or         Nigri-protein or     -   a vector according to the invention comprising at least two         nucleic acids comprising a pox-site or nox-site (preferably two         nucleic acids according to the invention that include a pox-site         or nox-site, respectively, flank a further DNA segment) and/or a         vector according to the invention comprising a nucleic acid         encoding for a Panto or Nigri-protein.

Explicitly included are non-human host organisms that only comprise a recombinant nucleic acid encoding for a Vika protein or Panto or Nigri protein respectively (and that do not comprise a nucleic acid including a vox site or pox site or nox site respectively). Furthermore, the invention includes non-human host organisms that only comprise at least one, preferably at least two, vox-sites or pox-sites or nox-sites respectively (and that do not comprise a nucleic acid encoding for a Vika protein or Panto or Nigri protein respectively). In that case, at least two vox sites (or pox site or nox site respectively). preferably flank another DNA segment. Upon cross-breeding of two non-human host organisms, wherein a first host organism comprises a recombinant nucleic acid encoding for a Vika protein (or Panto or Nigri protein respectively) and a second host organism comprises at least two recombinant vox-sites (or pox site or nox site respectively) and preferably flanking a further DNA segment, the offspring includes host organisms expressing Vika (or Panto or Nigri respectively) and further including the recognition sites vox (or pox or nox respectively), so that a site-specific DNA-recombination, like a tissue-specific conditional knock-out, is possible.

Non-human host organisms comprise a vector according to the invention or a nucleic acid according to the invention as described above that is, respectively, stably integrated into the genome of the host organism or individual cells of the host organism. Preferred host organisms are plants, invertebrates and vertebrates, particularly Bovidae, Drosophila melanogaster, Caenorhabditis elegans, Xenopus laevis, medaka, zebrafish, or Mus musculus, or embryos of these organisms.

The invention also includes a method for providing a non-human host organism, comprising the following steps:

-   -   providing a first non-human host organism comprising a nucleic         acid encoding for a Vika protein (preferably comprising a         nucleic acid sequence exhibiting at least 70%, preferably at         least 80%, preferably at least 90%, particularly preferred at         least 95%, even more preferred at least 99% nucleic acid         sequence identity to SEQ ID No. 3),     -   providing a second non-human host organism comprising at least         two nucleic acids, the nucleic acid sequence of which is         independently from each other comprising a vox site (a nucleic         acid sequence according to SEQ ID No. 2) or a nucleic acid         sequence reverse complementary thereto or a nucleic acid         sequence that is a functional mutant of the aforementioned         nucleic acid sequences, wherein the at least two nucleic acids         preferably flank another DNA segment (in particular a gene or a         promoter region),     -   cross-breeding of the first and the second non-human host         organism and from the offspring obtained thereby selecting the         non-human host organisms that comprise a nucleic acid encoding         for a Vika protein and at least two nucleic acids, the nucleic         acid sequence of which is independently from each other         comprising a nucleic acid sequence according to SEQ ID No. 2 or         a nucleic acid sequence reverse complementary thereto or a         nucleic acid sequence that is a functional mutant of the         aforementioned nucleic acid sequences.

Alternatively, the invention includes a method for providing a non-human host organism, comprising the following steps:

-   -   providing a first non-human host organism comprising a nucleic         acid encoding for a Panto protein or a Nigri protein,     -   providing a second non-human host organism comprising at least         two nucleic acids, the nucleic acid sequence of which is         independently from each other comprising a pox site (a nucleic         acid sequence according to SEQ ID No. 36) or a nox site (a         nucleic acid sequence according to SEQ ID No. 38) or a nucleic         acid sequence reverse complementary thereto or a nucleic acid         sequence that is a functional mutant of the aforementioned         nucleic acid sequences, wherein the at least two nucleic acids         preferably flank another DNA segment (in particular a gene or a         promoter region),     -   cross-breeding of the first and the second non-human host         organism and from the offspring obtained thereby selecting the         non-human host organisms that comprise a nucleic acid encoding         for a Panto or Nigri protein and at least two pox sites (in case         of Panto, the nucleic acid sequence of which is independently         from each other comprising a nucleic acid sequence according to         SEQ ID No. 36 or a nucleic acid sequence reverse complementary         thereto or a nucleic acid sequence that is a functional mutant         of the aforementioned nucleic acid sequences) or two nox sites         (in case of Nigri, the nucleic acid sequence of which is         independently from each other comprising a nucleic acid sequence         according to SEQ ID No. 38 or a nucleic acid sequence reverse         complementary thereto or a nucleic acid sequence that is a         functional mutant of the aforementioned nucleic acid sequences).

The invention provides a novel recombinase system suitable for producing a site-specific recombination in cells of various cell types. Thereby, a diverse range of genetic manipulations can be realized, particularly rearrangements of the DNA fragments flanked by vox sites (or pox sites or nox sites respectively) in same orientation (excision), opposite orientation (inversion) or when one vox site (or pox site or nox site respectively) is present on each of two DNA molecules with one if it being in circular form in any orientation (integration). Exemplary manipulations are the excision of a DNA segment that is flanked by two vox-sites (or pox sites or nox sites respectively) oriented in the same direction mediated by the Vika (or Panto or Nigri) recombinase. Amongst others, the recombinase systems according to the invention, in particular the Vika/vox system, provides the possibility to excise a vox-flanked (or pox-flanked or nox-flanked) stopper DNA fragment which is located 5′ of the gene and 3′ of the corresponding to the gene promoter. Without recombination the stopper sequence prevents gene expression, whereas upon recombinase-mediated (preferably Vika-mediated) excision of the stopper via two flanking recognition sites (preferably vox sites) the gene is located to the proximity of the promoter and therefore is expressed. Different types of promoter regions that regulate the expression of the recombinase (preferably Vika) allow, inter alia, conditional DNA recombination, when for example a tissue or organism-specific or inducible promoter region is used to express the recombinase (preferably Vika).

For the recombinase systems according to the invention, in particular the Vika/vox system, no cross-reactivity with other recombinase systems was observed. Therefore, the recombinase systems according to the invention, in particular the Vika/vox system, are applicable for use in combination with other recombinase systems and becomes a particular valuable tool for genetic experiments where multiple recombinases are required simultaneously or sequentially. In bacterial cells, in particular in E. coli, a similar level of activity compared to the Cre/loxP system could be demonstrated. In E. coli, the recombinase systems according to the invention, in particular the Vika/vox system, showed a significantly higher activity than the VCre/VloxP system. In addition, the recombinase systems according to the invention, in particular the Vika/vox system, is highly effective and specific in mammalian cells as well. In human and mouse cells Vika carries out site-specific recombination on vox-sites. In mammalian cells Vika has shown comparable activity to Cre and superior compared to VCre. Furthermore, the inventors demonstrate that Vika does not recombine pseudo-vox-sites from human or mouse genomes. As a consequence, and in contrast to Cre, there is no observed reduction in proliferation or observed cytotoxicity upon overexpression of Vika in these cells. Therefore, the recombinase systems according to the invention, in particular the Vika/vox system, is advantageous especially for those applications in higher organisms when by using conventional SSRs such as the Cre/lox system cytotoxicity is observed.

Due to its efficiency in a variety of cell types the recombinase systems according to the invention, in particular Vika/vox, can be used in particular for producing site-specific DNA recombination in cells in which other recombinase systems were shown to only achieve poor results. For example, one of the most widely used recombinase system Cre/loxP has limited application in some model organisms such as Caenorhabditis elegans, supposedly due to the presence of loxP-like nucleic acid sequences being naturally present in the genome of C. elegans. The inventors have shown that there is a significantly lower number of vox-like sequences for potential targeting by Vika in the genome of C. elegans. Therefore, recombination activity of Vika/vox in C. elegans is likely to be superior to Cre/loxP.

It was demonstrated by the inventors that only low numbers of so-called pseudo-recognition sites (pseudo-vox sites) of Vika are present in the human and mouse genome. The number of pseudo-vox sites is markedly lower than the number of pseudo-loxP sites (pseudo-recognition sites of Cre) in the mouse and human genome. It was shown, that Vika does not recombine on pseudo-vox sites originating from human and mouse genomes. On the contrary, Cre showed prominent activity on human and mouse pseudo-loxP sites.

Additionally, the inventors demonstrated in in vitro experiments, that stable expression of Vika does not cause cytophathic effects in human and mouse cells and that stable expression of Vika does not lead to increased DNA damage.

The object of the invention is also solved by the use of a protein with recombinase activity, wherein the protein comprises an amino acid sequence exhibiting at least 70%, preferably at least 80%, preferably at least 90%, particularly preferred at least 95%, even more preferred at least 99% amino acid sequence identity to one of the amino acid sequences according to SEQ ID No. 19, 21, 23, 25, 27, 29, 31 or 33 catalyze a site-specific DNA recombination. The aforementioned site-specific recombinase is used for site-specific DNA recombination at, preferably two, recognition sites that are identical or reverse complementary to each other, wherein at least one recognition site comprises a nucleic acid sequence according to or reverse complementary to the nucleic acid sequence according to SEQ ID No. 20, 22, 24, 26, 28, 30, 32 or 34, respectively; or a nucleic acid sequence that is a functional mutant thereof.

The object of the invention is also solved by the use of a protein with recombinase activity, wherein the protein comprises an amino acid sequence exhibiting at least 70%, preferably at least 80%, preferably at least 90%, particularly preferred at least 95%, even more preferred at least 99% amino acid sequence identity to one of the amino acid sequences according to SEQ ID No. 35 to catalyze a site-specific DNA recombination at, preferably two, recognition sites that are identical or reverse complementary to each other, wherein at least one recognition site comprises a nucleic acid sequence according to or reverse complementary to the nucleic acid sequence according to SEQ ID No. 36, respectively; or a nucleic acid sequence that is a functional mutant thereof.

The object of the invention is also solved by the use of a protein with recombinase activity, wherein the protein comprises an amino acid sequence exhibiting at least 70%, preferably at least 80%, preferably at least 90%, particularly preferred at least 95%, even more preferred at least 99% amino acid sequence identity to one of the amino acid sequences according to SEQ ID No. 37 to catalyze a site-specific DNA recombination. The aforementioned site-specific recombinase is used for site-specific DNA recombination at, preferably two, recognition sites that are identical or reverse complementary to each other, wherein at least one recognition site comprises a nucleic acid sequence according to or reverse complementary to the nucleic acid sequence according to SEQ ID No. 38, respectively; or a nucleic acid sequence that is a functional mutant thereof.

The proteins with recombinase activity and recognition sites are used in the following combination (table 1). Derivatives of the respective protein with amino acid sequence identities of at least 70%, preferably at least 80%, preferably at least 90%, particularly preferred at least 95%, even more preferred at least 99% and recognition sites with a nucleic acid sequence that is reverse complementary to the indicates nucleic acid sequence as well as their functional mutants are also included. It is preferred to use the indicated proteins with recombinase activity on wild type recognition sites or recognition sites with a nucleic acid sequence reverse complementary thereto.

TABLE 1 Amino acid sequence Nucleic acid Gen bank accession number of protein (site sequence of wild of protein; Organism specific recombinase) type recognition site EGU56467.1 SEQ ID No. 19 SEQ ID No. 20 Vibrio tubiashii ATCC 19109 YP_003065675.1 SEQ ID No. 21 SEQ ID No. 22 Methylobacterium extorquens DM4 YP_003280920.1 SEQ ID No. 23 SEQ ID No. 24 Streptomyces sp. W9 ZP_06822377.1 SEQ ID No. 25 SEQ ID No. 26 Streptomyces sp. SPB74 NP_395953.2 SEQ ID No. 27 SEQ ID No. 28 Agrobacterium tumefaciens str. C58 plasmid At YP_666181.1 SEQ ID No. 29 SEQ ID No. 30 Chelativorans sp_(—) BNC1 plasmid 3 YP_957160.1 SEQ ID No. 31 SEQ ID No. 32 Marinobacter aquaeolei VT8 plasmid pMAQU02 NP_943161.1 SEQ ID No. 33 SEQ ID No. 34 Pseudomonas sp. ND6 plasmid pND6-1 WP_008927019.1 SEQ ID No. 35 SEQ ID No. 36 Pantoea sp. aB YP_004250912.1 SEQ ID No. 37 SEQ ID No. 38 Vibrio nigripulchritudo

The invention is further based on the finding of the inventors that each of the proteins listed in table 1 in column 2, shows a Cre recombinase-like activity and recognizes recognition sites as indicated in table 1 in column 3. The organisms from which each of the site-specific recombinases is derived from are also indicated in table 1 (column 1). All of the proteins with recombinase activity listed in table 1 show a low amino acid sequence identity to Cre recombinase.

The invention further relates to a method for producing a site-specific DNA-recombination by contacting a protein with recombinase activity with the indicated recognition sites.

The invention also includes nucleic acids exhibiting a length not more than 40 base pairs, each nucleic acid comprising a nucleic acid sequence according to SEQ ID No. 20, 22, 24, 26, 28, 30, 32 or 34 or SEQ ID No. 36 or SEQ ID No. 38, a nucleic acid sequence that is a functional mutant thereof or a nucleic acid sequence reverse complementary thereto.

Additionally, the invention relates to vectors comprising said nucleic acid sequences, preferably at least one, even more preferred at least two identical or reverse complementary nucleic acid sequences.

In an even further embodiment the invention relates to vectors comprising a nucleic acid encoding for a protein with recombinase activity wherein the protein comprises an amino acid sequence exhibiting at least 70%, preferably at least 80%, preferably at least 90%, particularly preferred at least 95%, even more preferred at least 99% amino acid sequence identity to SEQ ID No. 19, 21, 23, 25, 27, 29, 31 or 33 or SEQ ID No. 35.

The use of any of the vectors according to the invention in a method for producing a site-specific DNA-recombination is also included in the invention.

Further, the invention includes an isolated host cell or an isolated host organism comprising

-   -   at least one, preferably at least two, nucleic acids according         to the invention comprising a recognition site as defined above         (preferably two nucleic acids according to the invention that         include a recognition site, respectively, flank a further DNA         segment) and/or a nucleic acid according to the invention         encoding for a protein with recombinase activity, as defined         above, or     -   a vector according to the invention comprising at least two         nucleic acids comprising a recognition site as defined above         (preferably two nucleic acids according to the invention that         include a recognition site, respectively, flank a further DNA         segment) and/or a vector according to the invention comprising a         nucleic acid encoding for a protein with recombinase activity as         defined above.

The embodiments of the invention that are described above in detail for site-specific recombination with Vika on vox-sites and Panto on pox sites and Nigri on nox sites are also included within the invention for site-specific recombination with one of the above mentioned site-specific recombinases on their recognition sites.

For each of the proteins with recombinase activity, the inventors identified numerous putative recognition sites with a lox-like structure in the organism wherefrom the recombinase was derived. Upon extensive studies only one thereof, the indicated respective wild type recognition site, turned out to be the actual recognition site of the indicated protein with recombinase activity. By expression of the nucleic acid sequence encoding for the site-specific recombinase in E. coli, the recombinase protein could be successfully obtained and its recombinase activity and specifity for the respective wild type recognition site could be demonstrated. It was shown that the indicated proteins with recombinase activity do not cross react with other known recombinase systems and are superior in the activity compared to some of the known recombinase systems, at least in certain cell types.

DESCRIPTION OF THE DRAWINGS

The invention is further illustrated by the following figures and examples without being limited to these.

FIG. 1 Amino acid sequence alignment of the sequences of Vika (SEQ ID No. 1) and Cre (SEQ ID No. 4). Residues from Cre-recombinase known to be essential for DNA interaction are highlighted, as well as their analogues in the recombinase Vika, with catalytic residues in black, DNA contacting residues underligned.

FIG. 2 A) Three-dimensional structure of the recombinase Vika obtained by 3D-modelling and its comparison with the crystal structure of B) Cre recombinase. Residues from Cre-recombinase known to be essential for DNA interaction are highlighted, as well as their analogues in the recombinase Vika, with catalytic and contacting residues.

FIG. 3 Nucleic acid sequence alignment of the recognition sites loxP (SEQ ID No. 5), rox (SEQ ID No. 9), vox (SEQ ID No. 2) and vloxP (SEQ ID No. 11).

FIG. 4 Determination of specificity of the Vika/vox system and analysis of potential cross-reactivities between Vika/vox and other recombinase systems when performing DNA recombination in E. coli. (−) and (+) indicate the addition of 100 μg/ml of L (+)-arabinose for the induction of DNA recombination. The bands for the non-recombinant plasmids are represented as two triangles, the recombinant plasmids are represented by one triangle, M . . . DNA marker.

A) Specificity of the indicated site-specific DNA recombinases Cre, Dre, VCre and Vika on vox sites, B) Specificity of the indicated site-specific DNA recombinases at recognition sites loxP, rox and VloxP.

FIG. 5 Determination of recombination activity of the indicated recombination systems Cre/loxP, VCre/VloxP and Vika/vox in E. coli. The bands for the non-recombined plasmids are represented as two triangles, the recombinant plasmids are represented by one triangle, M . . . DNA marker. Successful recombination events upon addition of various amounts of L (+)-arabinose for induction of DNA recombination are indicated: “no ind” . . . negative control without addition of L (+)-arabinose, further data show recombination upon addition of 1, 10, 100 μg/ml L (+)-arabinose as indicated.

FIG. 6 Determination of the recombination activity of the indicated recombination systems Cre/loxP, VCre/VloxP and Vika/vox in E. coli using a lacZ reporter assay. A) Schematic representation of the lacZ reporter assay; non-recombined plasmids express beta-galactosidase resulting in the formation of blue colonies when cultured on X-Gal-containing medium; in the recombined plasmids the promoter of the lacZ gene that was originally located between two recognition sequences was excised, thereby beta-galactosidase is not expressed, resulting in the formation of white colonies when cultured on X-Gal containing medium. B) Specificity of Cre, VCre and Vika in LacZ-based assay. White colonies, signifying recombination, only appear when a recombinase is expressed together with its corresponding reporter. Positive controls for the non-recombined (pSVpaX), and recombined form of the reporter (pSVpaXΔ) is shown as Mock.

FIG. 7 Comparison of recombination activities of the indicated recombinase systems Cre/loxP, VCre/VloxP and Vika/vox in human HeLa cells. A) Schematic representation of the EGFP-based reporter assays. The non-recombined plasmids express the gene for neomycin resistance (NeoR). Upon DNA recombination, the neomycin cassette is removed so that the cytomegalovirus promoter (CMV prom), that was originally driving expression of the neomycin resistance gene, is located to the proximity of the EGFP gene induces its expression. B) % GFP-positive cells resulting from the recombination assays for Cre, VCre and Vika in HeLa cells. C) Fluorescence microscopic image of the recombination assays for Cre, VCre and Vika in HeLa cells.

FIG. 8 Evaluation of genotoxicity of Vika. Recombination specificity of Vika and Cre on the respective cryptic human and mouse chromosomal target sites (see example 6). − and + indicates presence or absence of L(+)-arabinose (100 μg/ml) in the growth medium. Non-recombined and recombined plasmids are denoted as two triangles and one triangle, respectively. M, marker, 2-log DNA ladder, NEB.

FIG. 9 Evaluation of prolonged expression of Vika in mouse ES cells. A) mouse ES cell line with stably integrated Vika recombinase after prolonged passaging (24 days). A representative photo of a clonal culture is depicted (brightfield image). B) Recombination activity of stably expressed Vika recombinase in mouse ES cell line. Images show cells 24 hours after transfection with vox-GFP reporter plasmid. Note the apparent Vika-mediated recombination signified through GFP expression. A control of the recombined reporter plasmid (GFP+) was transfected for detecting transfection efficiency.

FIG. 10 Evaluation of DNA damage induction in mouse NIH3T3 cells by γ-H2AX assay 72 h after infection with indicated recombinase-expressing virus. Quantification of the γ-H2AX positive cells either infected with virus for respective recombinase expression or treated with camptothecin for 2 h. Statistically significant increase of γ-H2AX signals is indicated by asterisks. Error bars indicate standard deviation of the mean value. (** indicate p=0.01). n=3.

FIG. 11 Evaluation of cytotoxicity of Vika. Proliferation effects upon expression of indicated recombinases in mouse NIH3T3 cells. Cells were infected with bicistronic retroviruses expressing respective recombinase linked to GFP. Every 72 hours cells were analyzed by flow cytometry. Error bars indicate standard deviation of the mean value, n=2.

FIG. 12 Recombinase activity of Panto and Nigri on pox and nox recombination target sites in E. coli

EXAMPLE 1 3D Model of a Protein with Recombinase Activity (Vika)

Vika is annotated in NCBI under number ZP_(—)05884863 and originates from the gram-negative bacterium Vibrio coralliilyticus ATCC BAA-450. Vika exhibits a low amino acid sequence identity of 27.7% to Cre recombinase (50.2% sequence similarity) (FIG. 1). In order to analyze the binding properties of the protein ZP_(—)05884863 compared to Cre recombinase, a 3D atom model was developed using the crystal structure of Cre recombinase as a template (FIG. 2). The resulting 3D model showed an RMSD (root-mean-square deviation) of 2.4±0.3 Å when compared to the Cre template. From this data a strong structural similarity between these two proteins can be concluded. It is apparent from the 3D model that five catalytically important residues known from Cre recombinase are conserved in Vika.

In extensive experiments six putative recognition sites from the genome of Vibrio coralliilyticus ATCC BAA-450 were identified by the inventors. Therefrom, the nucleic acid herein referred to as vox-site was identified to be the recognition site of Vika. The vox site is a 34 bp DNA sequence consisting of two inverted repeats, comprising about 50% sequence homology to loxP and 55% to VloxP and about 33% sequence homology to rox (FIG. 3). The nucleic acid sequence of the vox-site is presented in SEQ ID No. 2.

EXAMPLE 2 Recombinase Activity of Vika and Recognition of Vox-Sites in E. coli

To verify whether the protein Vika exhibits recombinase activity and to verify that vox is its recognition site, a nucleic acid encoding for Vika was cloned into an E. coli recombination reporter plasmid that comprised two vox-sites of the same orientation. The recombination reporter plasmid was based on the plasmid pEVO (Buchholz and Stewart, 2001), in which the recombinase was inserted via a BsrGI and XbaI cleavage site. In the plasmid, the two vox-sites flanked an approximately 1 kb DNA segment that was excised by DNA-recombination. DNA recombination was induced by the addition of L (+)-arabinose. It was shown that Vika mediated a DNA recombination at vox-sites (FIG. 4A, right). Therefore, it could be shown that the Vika/vox system is a recombinase system applicable in E. coli cells.

In further experiments, the activity of Vika/vox in different cell types as well as possible cross-reactions with other recombinase systems were assessed.

EXAMPLE 3 No Cross-Reactivity with Other Recombinase Systems—No Recognition of Other Lox Sites by Vika and No Recognition of Vox Sites by Other Recombinases

To assess whether Vika recognizes the recognition sites of known recombinase systems, in particular loxP, VloxP and rox, the nucleic acid sequence encoding for Vika was cloned into E. coli recombination reporter plasmids comprising the aforementioned recognition sequences, respectively. The recombination reporter plasmids were based on the plasmid pEVO (Buchholz and Stewart, 2001), into which the recombinase was inserted via a BsrGI and XbaI cleavage sites.

In a reporter plasmid, an approximately 1 kb long DNA portion was flanked by two lox sites of the same orientation (either loxP, VloxP or rox). Thereby upon site-specific DNA recombination (induced by addition of L (+)-arabinose) the 1 kb DNA segment was excised from the plasmid. FIG. 4B shows that the recombinase Vika is not applicable to produce a DNA recombination at any of the lox sites loxP, VloxP and rox.

Furthermore it was assessed whether other known recombinases can produce a site-specific recombination on vox sites. For this purpose, reporter plasmids, each containing Cre, Dre, VCre or Vika and further including two vox sites were transformed into E. coli. The results show that only Vika, but not Cre, Dre and VCre induce recombination at vox-sites (FIG. 4A). Further experiments using lacZ reporter assays confirmed the specificity of Vika to its recognition sites vox.

Therefore, Vika can be used in combination with other recombinase systems without causing cross-reactions.

EXAMPLE 4 Activity of Different Recombinase Systems in E. coli

The activity of the recombinase systems Vika/vox, Cre/loxP and VCre/VloxP was compared in E. coli cells. For this purpose recombination reporter plasmids as described in Example 2 or 3 that included the nucleic acid sequence encoding for the respective recombinase and two respective recognition sites of the same orientation were used. For a quantitative analysis, different concentrations of L (+)-arabinose were added in order to induce the DNA recombination: 0, 1, 10 and 100 μg/ml (FIG. 5).

Further, in another approach, a lacZ reporter assay was used in order to compare the activities of the three above-mentioned recombinase systems. For this purpose, reporter plasmids were constructed by introducing two of the respective recognition sequences (vox, 10× and VloxP) into a pSV-paX1 vector ((Buchholz and Bishop, 2001), schematic representation in FIG. 6A). Plasmid DNA was isolated and introduced into DH5α cells by electroporation. The resulting cells were used in defined concentration and plated in the presence of ampicillin on X-gal containing plates. The number of white and blue colonies was counted (FIG. 6B).

In a further approach using two vectors comprising one recognition site (loxP, VloxP or vox), respectively, and the corresponding recombinases (Cre, VloxP, Vika) it was shown that only combinations of two identical target sites and their corresponding recombinases produced co-integrant plasmids.

EXAMPLE 5 Activity of Different Recombinase in Mammalian Cells

To assess out whether the Vika/vox system is active in cells other than the natural host cells and E. coli, recombinase activity was analyzed in human HeLa cells and compared with activities of the known recombinase systems Cre/loxP and VCre/VloxP.

For this purpose, HeLa cells were transfected with a reporter plasmid encoding for EGFP. The reporter plasmids were based on the plasmid pEGFP-X ((Buchholz and Bishop, 2001), schematic representation in FIG. 7A). A neomycin cassette was flanked by two of the respective recognition sites of the same orientation. Using this plasmid, cloning of the respective recognition sites into said plasmid, the reporter plasmids pRK5-loxP-EGFP, pRK5-VloxP-EGFP and pRK5-vox-EGFP were derived. HeLa cells were plated in 6-well plates with a cell number of 2×10⁵ cells and cultured in cell culture medium (4.5 mg/ml glucose DMEM comprising 10% FBS and 100 U/ml penicillin/streptomycin). The reporter plasmids comprising the recognition sites (pRK5-loxP-EGFP, pRK5-VloxP-EGFP and pRK5-vox-EGFP) were co-transfected into HeLa cells with the respective recombinase expression plasmid pNPK-NLS-Cre, pNPK-NLS-VCre and pNPK-NLS-Vika. Cells were cultured for 24 h. Subsequently, the cells were washed with PBS and fixed and examined using fluorescence microscopy (FIG. 7B, C).

It could be shown that the Vika/vox system is suitable for producing a DNA recombination in human HeLa cells and that it shows superior activity when compared to the known VCre/VloxP system.

EXAMPLE 6 Activity on Cryptic Chromosomal Site of Mouse and Human Genomes

In order to investigate potential site-effects of Vika, pseudo-vox sites in the mouse and human genome were identified. In both species, lower overall numbers of pseudo-vox sites were uncovered than pseudo-loxP sites. Pseudo-vox sites that most closely resembled vox sites were tested experimentally. The nucleic acid sequences of the tested pseudo-vox sites correspond to the following SEQ ID No.:

-   -   voxCH18 (7 mutations): SEQ ID No. 12     -   voxCH21 (6 mutations): SEQ ID No. 13     -   voxCHX (6 mutations): SEQ ID No. 14     -   voxCMp92 (6 mutations): SEQ ID No. 15     -   voxCHp3 (6 mutations): SEQ ID No. 16

As a comparative example, recombination with Cre on two pseudo-loxP sites was examined. The nucleic acid sequences of the tested pseudo-loxP sites correspond to the following SEQ ID No.:

-   -   loxhXp22 (7 mutations): SEQ ID No. 17     -   loxM5 (5 mutations): SEQ ID No. 18

It was demonstrated that Cre showed prominent activity on pseudo-loxP sites (FIG. 8) in E. coli. In contrast, Vika did not display measurable activity on pseudo-vox sites in these assays (FIG. 8).

EXAMPLE 7 Prolonged Constitutive Expression in Primary Cells (Mouse ES Cells)

In order to examine the influence of prolonged expression of Vika in primary cells, mouse ES cells stably expressing Vika were generated. No effect on cell growth and morphology was observed, indicating that Vika is active when selected for stable expression in mouse ES cells and is well tolerated (FIG. 9).

EXAMPLE 8 DNA Damage Evaluation Based on γ-H2AX Assay

The potential impact of recombinase expression on DNA damage was examined. NIH3T3 cells were infected with GFP-bicistronic retroviral particles encoding Cre, Vika or controls. Three days post infection, γ-H2AX signals were investigated in fixed cells. Cre expression caused a marked increase in γ-H2AX signals signifying induction of DNA damage. In contrast, Vika had no influence on the amount of 7-H2AX counted (FIG. 10), indicating that Vika expression does not lead to increased DNA damage in these cells.

EXAMPLE 9 No Cytotoxic Effect Upon High-Level Expression in Mouse Cells. (Retroviral Experiment). (FIG. 11)

The percentage of GFP-positive cells over time in the populations of example 8 was examined as an indicator for cytotoxicity. Cre expression led to a rapid decline of GFP positive cells in the population (that is consistent with previously published information). However, no decline in GFP positive cells was observed in Vika expressing cells (FIG. 11). These experiments demonstrate that high levels of Cre are cytotoxic, whereas Vika expression is well tolerated in the same setting.

EXAMPLE 10 Recombinase Activity of Panto and Nigri on Pox and Nox Recombination Target Sites in E. coli

To verify whether the protein Panto and Nigri exhibit recombinase activity and to verify that pox and nox are corresponding recognition sites, a nucleic acid encoding for Nigiri was cloned into an E. coli recombination reporter plasmid that comprised two nox-sites of the same orientation. A plasmid containing gene coding for Panto protein and two identical sequences of pox site was created in a similar way. The recombination reporter plasmid was based on the plasmid pEVO (Buchholz and Stewart, 2001), in which the recombinase was inserted via a BsrGI and XbaI cleavage site. In the plasmid, the two vox-sites flanked an approximately 1 kb DNA segment that was excised by DNA-recombination (FIG. 12). DNA recombination was induced by the addition of L (+)-arabinose. It was shown that Vika mediated a DNA recombination at pox-sites Panto and Nigri on nox sites. Therefore, it could be shown that the Panto/pox and Nigri/nox system are recombinase systems applicable in E. coli cells.

CITED NON-PATENT LITERATURE

-   Buchholz, F., and Bishop, J. (2001). loxP-directed cloning: Use Cre     recombinase as a universal restriction enzyme. BioTechniques 31,     906-. -   Buchholz, F., and Stewart, A. F. (2001). Alteration of Cre     recombinase site specificity by substrate-linked protein evolution.     Nat Biotechnol 19, 1047-1052. -   Chung Y et al. Cell Stem Cell 2008 (2): 113-117, and supplemental     material -   Lin et al. Cell Res 2007; 17:999-1007 -   Mai et al. Cell Res 2007 17:1008-1019 -   Suzuki, E., and Nakayama, M. (2011). VCre/VloxP and SCre/SloxP: new     site-specific recombination systems for genome engineering. Nucleic     Acids Research 39, e49-e49. 

1. A method for producing a site-specific DNA-recombination, comprising: contacting a protein with recombinase activity inside a cell, wherein the protein exhibits an amino acid sequence of at least 70%, preferably at least 80%, preferably at least 90%, particularly preferred at least 95%, even more preferred at least 99%, amino acid sequence identity to SEQ ID No. 1 with at least two recognition sites that are identical or reverse complementary to each other, wherein at least one recognition site comprises a nucleic acid sequence according to or reverse complementary to SEQ ID No. 2, wherein upon binding of the protein with recombinase activity to the two recognition sites site-specific DNA-recombination occurs.
 2. The method according to claim 1, further comprising: introducing into the cell a nucleic acid encoding for the protein with recombinase activity.
 3. The method according to claim 1, wherein the cell comprises a nucleic acid encoding for the protein with recombinase activity, preferably as defined in claim 2, or method according to claim 2, wherein the nucleic acid encoding for the protein with recombinase activity comprises a regulatory nucleic acid sequence and expression of the nucleic acid encoding for the protein with recombinase activity is produced by activating the regulatory nucleic acid sequence.
 4. The method according to claim 1, 2, or 3, wherein the cell is selected from eukaryotic or bacterial cells.
 5. (canceled)
 6. Use of a protein with recombinase activity, wherein the protein comprises an amino acid sequence exhibiting at least 70%, preferably at least 80%, preferably at least 90%, particularly preferred at least 95%, even more preferred at least 99% amino acid sequence identity to SEQ ID No. 1 to catalyze a site-specific DNA-recombination at recognition sites that are identical or reverse complementary to each other, wherein at least one recognition site comprises a nucleic acid sequence according to or reverse complementary to SEQ ID No.
 2. 7. (canceled)
 8. A nucleic acid exhibiting a length not more than 40 base pairs, the nucleic acid comprising a nucleic acid sequence according to SEQ ID No. 2 or a nucleic acid sequence reverse complementary thereto.
 9. (canceled)
 10. A vector comprising at least one, preferably at least two, identical or reverse complementary nucleic acids according to claim
 8. 11. The vector according to claim 10, wherein a further DNA segment is flanked by two identical or reverse complementary nucleic acids according to claim
 8. 12. Use of a vector comprising a nucleic acid encoding for a protein with recombinase activity wherein the protein comprises an amino acid sequence exhibiting at least 70%, preferably at least 80%, preferably at least 90%, particularly preferred at least 95%, even more preferred at least 99% amino acid sequence identity to SEQ ID No. 1 in a method according to claim 2, 3, or
 4. 13. Use of a vector according to claim 10 or 11 in a method according to claim 2, 3, or to
 4. 14. An isolated host cell or non-human host organism, comprising the following recombinant DNA fragments: (a) at least one, preferably at least two, nucleic acids according to claim 8; and/or a nucleic acid as defined in claim 12, or (b) a vector according to claim 10 or 11; and/or a vector as defined in claim
 12. 15. (canceled) 